feat: implement core markdown rendering pipeline

Add minimal e2e pipeline to transform content → HTML:

- src/error.rs: Custom error types with thiserror
- src/content.rs: YAML frontmatter parsing via gray_matter
- src/render.rs: Markdown → HTML via pulldown-cmark
- src/templates.rs: Maud templates for base layout and posts
- src/main.rs: Pipeline orchestrator

All 15 blog posts successfully rendered to public/blog/*/index.html.
Added thiserror and walkdir dependencies.
Added public/ and DepMap fragment to .gitignore and AGENTS.md.

AGENTS.md

@@ -21,6 +21,7 @@ The agent MUST read and adhere to the global engineering ruleset and any active
 **Active Fragments:**
 
 - Rust idioms (`.agent/predicates/fragments/rust.md`)
+- DepMap MCP tools (`.agent/predicates/fragments/depmap.md`)
 
 **Available Workflows:**